The present invention relates to a motor speed control method and apparatus having a drooping function to effect speed control.
When a material to be rolled is rolled by using a plurality of motors as in a steel rolling line, it is demanded to improve the degree of coincidence of speed (speed matching) or load balance among motors in order to prevent the tension applied to the material to be rolled from changing.
There has been known a system in which speed control apparatuses are provided with drooping functions so that motors may have drooping characteristics to improve speed matching among motors. In conventional drooping characteristics, a drooping variable proportionate to the load current of the motor is derived and subtracted from a speed command value, whereby the motor speed is lowered in proportion to the load current of the motor. At the time of acceleration or deceleration of the motor, however, the drooping variable is derived from not only the load current but also the acceleration or deceleration current. In general, the acceleration or deceleration currents of respective motors differ because of a difference in moment of inertia or the like between motors. Even if drooping characteristics of motors are equally matched, therefore, speed drooping variables of motors differ at the time of acceleration or deceleration. As a result, speed matching of motors is disturbed.
In a known solution to this, the motor current is separated into a load current and an acceleration or deceleration current and the drooping variable (speed drooping variable) is derived from the load current alone. This is described in JP-A-57-65288, for example. Separation into the load current and the acceleration or deceleration current is conventionally conducted by deriving the acceleration or deceleration current from the voltage applied to the motor by means of simulation and subtracting the computed value of the acceleration or deceleration current from the actual value of the motor current.